maint.mk: improve "can not" detection and generalize rule name

maint.mk: improve "can not" detection and generalize rule name

* top/maint.mk (sc_prohibit_misc): Renamed from sc_prohibit_can_not,
since we'll probably add a few more word pairs here.
Use the same technique as in sc_prohibit_doubled_word, so that
we recognize "can not" also when the words are separated by a newline.
Suggested by Eric Blake.
(perl_filename_lineno_text_): Define.  Factored out of...
(prohibit_doubled_word_): ...here.  Use the new definition.
(prohibit_misc_): New var.  Use it here, too.
(prohibit_misc_RE_): New overridable variable.